By default, an archetype cannot run on a project that already exists. However, it's possible to allow an archetype to run on a pre-existing project by adding an allowPartial element set to true in your archetype's archetype.xml file.

As an example, here is an archetype project called "maven-archetype-bacon-and-eggs":

The "maven-archetype-bacon-and-eggs" project's archetype.xml file is shown here. Notice that it contains an allowPartial element set to true. It also specifies two resources for the archetype, a bacon.txt file and an eggs.txt file.